1) Specific heat capacity and heat capacity refer to the amount of heat required to raise an object's temperature by 1 degree Celsius. Water has a high specific heat capacity of 4.18 kJ/kg°C. 2) Latent heat refers to the heat absorbed or released during phase changes like melting or vaporization without changing temperature. The latent heat of vaporization for water is 2.26 MJ/kg at 100°C. 3) In anesthesia, latent heat effects are important for vaporizers and gas storage. Rapid gas use can lower cylinder temperatures through latent heat absorption from the remaining liquid. Proper heating maintains safe pressures.

1. Heat Capacity and Latent Heat
MOHAMED ANWER RIFKY

2. 1-Specific heat capacity is defined as the amount of heat required to raise the temperature
of 1
kilogram of a substance by 1 kelvin (SI unit of specific heat capacity J kg-1
K-1).
2-Heat capacity is defined as the amount of heat required to raise the temperature of a given
object by 1
kelvin (SI unit of heat capacity J K-1).
**N.B 1 K is commensurate with
1°C.
A-In a Vaporizer (component parts of different materials>>calculated heat
capacity).
B-Tissue specific heat capacities (mean value of 3.5 kJ kg-1 °C-1 >>a 70 kg patient (
245 kJ °C-).
C-I f temperature has fallen to 36°C>>shivering >>increase the heat production four folds to 320 W (or 240 x
60 joules
per minute, i.e. 14.4 kJmin-1.) The patient ought to shiver
for
17 minutes to produce the heat
required.
**Both core temperatures and surface skin temperatures
forms
>>mean body temperature.
D-The specific heat of water is 4.18 kJ
Kg-1 °C_1. E-1 calorie being equal to 4.18
joules.
F-In dietetics the kilocalorie is often written as Calorie with a capital
C.
G-The specific heat of most other substances is less than that of water.The high
specific
heat of water is used to provide a reservoir of heat
(vaporizers ).
H- 2 kg of blood ( 2 litres), are transfused at 5°C and warmed up to 35°C in the patient.
The heat required = 2 kg x 3.6 kJ kg-1= (2 x 3.6 x 30) kJ=
216 KJ
So, the patient's mean temperature must fall by up to 1°C when 2
litre of unwarmed blood are transfused>>
>Blood warmers.
I-Gases have a low specific heat (1.01 kJ kg-1 °C_1) at constant
pressure.
1.2 J litre-1 °C_1, (
a
Thousandth) of the numerical value when expressed in terms of
mass.
a-An extremely small quantity of heat is required or lost when the temperature of
a small
volume of a gas is altered.
b- Gases pass along the anaesthetict ubing, only minimal quantities of heat are
transferred >>
gas is closer to ambient
temperature >>
at the patient (humidifiers
!!).
c-Heat loss from warming inspired air = Flow X Specific heat capacity X Temperature rise(20 to 34 c.) = 7 litre min-1 x 1.2 J.litre-1 °C-1 x 14°C= 118 J min-1= 1.96 W (1
W = 1 J s--1).
So,it is only about 2 W (Of totall of 80 W.)>> not normally an important factor.
3-LATENT HEAT: When a substance changes from a liquid to a vapour or from a solid to a liquid, heat must be supplied (at a constant
temperature).
latent heat of fusion is when a solid changes to a liquid. Solid out from a liquid (latent heat of crystallization).
The latent heat of vaporization is of most
interest in
anaesthesia.
Specific latent heat is defined as the heat required to convert 1 temperature (SI unit of specific latent heat
Jkg-1).
kilogram of a substance from one phase to another at a
given
A-Thus, water at 100°C may be converted to steam at 100°C by supplying the appropriate quantity of latent heat, i.e. 2.26
MJkg-1.
B-At body temperature it is found that 2.42 MJ are required to turn 1 kg water into 1 kg vapour (The lower the
temperature
the more latent heat is needed to vaporize a
substance).
latent heat would continue
to fall as temperature
rose>>ultimately reach
C-The temperature at which the latent heat of vaporization of nitrous oxide becomes zero =36.5°C( a
temperature where the substance changes spontaneously from liquid to vapour without the supply of any
external energy).
4-LATENT HEAT IN ANAESTHESIA:
1-Vaporization of the ethyl chloride >>pronounced cooling of the skin thus impairing conduction >>
abscess or whitlow).
analgesia (For the opening of a skin
2-Most modern vaporizers now have systems for controlling the concentrations of vapour based on thermostatic devices (fall in
temperature of the anaesthetic in the vaporizer >>less volatile).
3-If a nitrous oxide cylinder is allowed to empty rapidly >>converted to gas, the latent heat required being taken from the remaining fluid
and from
the cylinder walls>>Temp.falls and water vapour from the air may condense or freeze on the outside of it.

3. 4-As a result of cooling, the N2O vapour pressure falls rapidly inside (the pressure gauge)>>low reading which recovers towards
the
previous level after the cylinder is turned off (the dotted line in the fig.).
5-Carbon dioxide and cyclopropane are also stored in liquid form(in normal anaesthetic practice the rate of use is slow that
cooling of the
cylinder is not noticed.
6-In practice, the liquid oxygen in the storage vessel is at a temperature of around -160°C, and must be kept in
a special storage vessel resembling a gigantic vacuum flask to maintain it at this low temperature (N.B O2
critical temp. is -119 C.).
1-When oxygen is taken from the top of the storage vessel, it is very cold
>>superheater coil---A pressure regulator (at about 4.1 bar).
2-If oxygen flows at a fast rate>>latent heat is taken and >> vapour pressure falls>>
a supplementary source of heat is needed (a pressure-raising vaporizer). A control valve
senses the storage vessel pressure and controls the flow of liquid oxygen to the
pressure-raising vaporizer(the lower the pressure, the higher the flow of liquid oxygen).
In the vaporizer the oxygen is warmed and vaporized to the pipeline pressure.
3-If no oxygen is used ( Temperature of the storage vessel gradually rises )>>
oxygen to blow off through the safety valve ,then this liquid oxygen then reduces
the temperature and pressure.
5-LATENT HEAT AND HEAT LOSS FROM THE PATIENT:
1-Humidity in the upper trachea = 34 mg litre-1
2-Total water required to humidify dry air = Specific latent heat of vaporization at 37°C X Total water
Specific heat to warm inspired air (calculated previously)= 2.0 W
3-Total heat loss from respiration = 11.6 W
In anaesthesia, however,the inspired gases are usually dry and consequently more heat is lost by this
route than under normal conditions(15% of the total basal heat loss of about 80 W.Instead of the 10%
given in Chapter 9).
4-The older closed anaesthetic system with the Waters canister >> efficient conservation of a
patient's heat and moisture other sources of heat loss are blocked by placing the person in a wax
bath,and as a in the treatment of cancer.